1. Field of the Invention
The present invention relates to inkjet printing methods and inkjet ink sets using white inkjet ink.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
When jetting the inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes or pigments and polymeric binders. It will be readily understood that the optimal composition of such ink is dependent on the printing method used and on the nature of the ink-receiver to be printed. The ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        solvent-based, the drying primarily involving evaporation;        oil-based, the drying involving absorption and penetration;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and        UV-curable, in which drying is replaced by polymerization.        
It should be clear that the first three types of ink compositions are more suitable for a receiving medium that is more or less absorptive, whereas hot melt inks and UV-curable inks are usually printed on non-absorbent ink-receivers.
White inkjet inks are generally used for so-called “surface printing” or “backing printing” to form a reflection image on a transparent substrate. In surface printing, a white background is formed on a transparent substrate using a white ink and further thereon, a color image is printed, whereafter the formed final image is viewed from the printed face. In so-called backing printing, a color image is formed on a transparent substrate using color inks and then a white ink is applied onto the color inks, and the final formed image is observed through the transparent substrate. In a preferred embodiment the colour inkjet ink is jetted on partially cured white inkjet ink. If the white ink is only partially cured, an improved wettability of the colour ink on the white ink layer is observed. Partially curing immobilizes the ink on the substrate surface. A quick test for determining if the white inkjet ink is partially cured can be done by rubbing a finger or a cloth across the printed surface, whereby it is observed that ink can be smeared or smudged on the surface.
Pigments with a high refractive index, such as titanium dioxide, are used in the white ink in order to obtain a sufficient opacity of the printed layer. Sedimentation of these dense particles in a low viscosity fluid, such as an inkjet ink, is a real challenge for ink formulators. Problems of clogging of inkjet printhead nozzles and poor storage stability of the ink are direct consequences of sedimentation and aggregation of white pigments due to the difference in specific gravity between pigment particles and the liquid medium of the ink.
Various approaches have been used trying to overcome these problems. One approach is to improve the dispersibility. EP 1388578 A (DAINIPPON INK) discloses an ultraviolet-curable ink composition for inkjet recording including titanium oxide, a polymeric dispersant having a basic functional group, a photopolymerizable compound and a photopolymerization initiator, the titanium oxide is surface-treated with silica and alumina and the weight of the silica, which coexists with the titanium oxide, is larger than that of the alumina.
Another approach is designing particles which exhibit less sedimentation. U.S. Pat. No. 4,880,465 (VIDEOJET) discloses a non-pigmented white inkjet ink including hollow microspheres containing a central void region filled with a liquid capable of diffusing through the walls of the microspheres and have an inside diameter from about 0.1 to about 0.5 micron and an outside diameter from about 0.4 to about 1 micron. Sedimentation is drastically reduced but the opacity of such a printed white layer remains limited.
A third approach is the adaptation of the hardware involving an agitating device for reducing sedimentation, such as e.g. a stirrer in the supply vessel of the white ink. However it is complex and costly to foresee an agitating device in the printhead and the tubing from the supply vessel to the printhead.
EP 1803781 A1 (KONICA) discloses an active ray curable ink-jet ink set including colour inks and a white ink based on titanium dioxide.
WO 2007/035505 A1 (DU PONT) discloses an aqueous white ink including a polymerically dispersed titanium dioxide and a crosslinked polyurethane binder.
EP 1818373 A2 (FUJIFILM) discloses an inkjet ink composition including: a white pigment; a polymerizable compound; and a polymerization initiator wherein the white pigment includes at least one of inorganic hollow particles or inorganic-organic hybrid hollow particles.
It would be desirable to be able to print white layers of consistent quality on a wide variety of ink-receivers using a state-of-the-art inkjet printer not requiring any complex or costly adaptation of the printer, wherein sedimentation problems of the pigment in the white inkjet ink are strongly reduced or eliminated.